This invention relates to a miniaturized zoom lens system having a super wide viewing angle of 84.degree. or 94.degree. and good compensation for various aberrations. These criteria are achieved by, inter alia, providing one of the lenses with an aspherical surface.
The lens system of the present invention is a so-called two lens group type of zoom lens system comprising a first divergent lens group and a second convergent lens group. The movement characteristics of the two lens groups are illustrated in FIG. 1, where f.sub.1 (f.sub.1 &lt;0) is the focal length of the first lens group, f.sub.2 (f.sub.2 &gt;0) is the focal length of the second lens group, and l is the distance between the first and second lens groups. The relation of these parameters to the overall focal length f is as follows: ##EQU1##
The back focal length f.sub.B is given by: ##EQU2##
From equation (1): ##EQU3##
From equation (2): ##EQU4##
The overall length L is: EQU L=l+f.sub.B ( 5)
From equations (3), (4) and (5): ##EQU5##
In equation (6), if dL/df =0, then EQU f=.vertline.f.sub.1 .vertline. (7)
From equation (3), the distance l between the first and second lens groups is longest in the wide angle position, and the longer the overall focal length f the shorter the distance l becomes. From equation (4), the back focal length f.sub.B has a minimum value in the wide angle position, and the longer the overall focal length the longer the back focal length proportionately becomes. From equation (7), the overall length L has a minimum value when the overall focal length f is equal to the absolute value .vertline.f.sub.1 .vertline. of the focal length of the first lens group. Accordingly, whenever the overall focal length f is longer or shorter than .vertline.f.sub.1 .vertline., the overall length L becomes longer.
From the above analysis, since the distance l between the first and second lens groups becomes longest at the wide angle position, the aperture of the first lens group tends to increase as the viewing angle widens. This tendency is effective to extremely widen the viewing angle. If the miniaturization of the lens system is unreasonably compatible with widening the viewing angle to a super-wide angle, the barrel shaped distortion aberration in the wide angle position is abrupty increased, and it is impossible to compensate for this aberration by using a spherical lens system.